Error 4521 (elements perpendicular to their coordinate system x-axis)

[Gabriel93]

Hello,

I have a model that I received as a .nas/.bfd file. When I try to run modal analysis on it, I have in the f06 file fatal errors on 17 elements (the model has a total of 719674 elements) saying:
USER FATAL MESSAGE 4521 (MC2THETA)
THE X-AXIS OF COORDINATE SYSTEM ... IS PERPENDICULAR TO ELEMENT ...
The explanation of this error is "The user supplied material orientation coordinate system x-axis is normal to the plane of the element and, hence, the material orientation angle with lines = constant of the element is indeterminate. Choose a new coordinate system ID whose x-axis is not normal to the plane of the element.', which I don't understand
And for the 17 elements, that are in the same area, the error is with the same coordinate system.
Do you know what should I work on to solve this error ? How can I modify either the elements or the coordinate system itself to solve it ?

Thank you